added screenshot for learning brain in prefab

docs/Basic-Guide.md

    **Note**: The platforms in the `3DBall` scene were created using the `Platform` prefab.  Instead of updating all 12 platforms individually, you can update the `Platform` prefab instead.
    
    ![Platform Prefab](images/platform_prefab.png)
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+   
+   ![3dball learning brain](images/3dball_learning_brain.png)
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 4. Make sure that all of the Agents in the Scene now have **3DBallLearning** as `Brain`.
    __Note__ : You can modify multiple game objects in a scene by selecting them all at 
    once using the search bar in the Scene Hierarchy.